There is an interest in digital output image sensors as an imaging device which generates high-definition videos and still images. The digital output image sensor includes, for example, an imaging area having pixels disposed in rows and columns, a row selection circuit, a horizontal scan circuit, and an analog-to-digital conversion circuit which performs an analog-to-digital conversion on an analog pixel signal output from the imaging area into a digital output signal. The digital output image sensor employs, for example, a column parallel analog-to-digital conversion scheme in which an analog-to-digital converter is disposed for each column of pixels. In other words, analog-to-digital converters as many as the number of columns of pixels are disposed in the column parallel analog-to-digital conversion scheme. Thus, the number of analog-to-digital converters increases to achieve higher-definition and a larger number of pixels. This requires each analog-to-digital converter to generate a more precise digital output signal at a higher speed.
Patent Literature (PTL) 1 discloses a configuration of a solid-state imaging device which includes, to achieve a high-speed digital output image sensor, an analog-to-digital conversion circuit in which cyclic analog-to-digital converters are serially connected in two stages. According to this, one cyclic analog-to-digital converter generates digital values of more significant bits, and another cyclic analog-to-digital converter generates digital values of less significant bits. Thus, processing of reading out an analog pixel signal and analog-to-digital conversion process can be performed in parallel, thereby allowing a high-speed digital output image sensor.